Eddy Current Losses in Large Air Coils With Layered Stranded Conductors

A procedure based on the finite-element method (FEM) is presented to determine the eddy currents and losses in large axisymmetric filter air coils built with a special type of turns. To reduce the eddy current losses, the turns consist of several single insulated copper wires which are twisted similarly to a cable to form a conductor. Therefore, axisymmetry in the cable is destroyed. Since the wires are not stranded perfectly within the cable (they are stranded within layers only), there is a non-uniform current distribution between the different layers. To take this into account and still retain axisymmetry, an FEM procedure using a one component vector potential combined with circuit considerations has been used where the single wires of the cable are located in a special manner to simulate their stranding within their respective layers. The numerical results are validated by measurements showing excellent agreement.